Recent studies in my laboratory and others have illustrated that the peri-adolescent rat differs markedly in its behavior and in its sensitivity to psychopharmacological agents when compared with younger or older animals. Animals of this age show an attenuated behavioral response to drugs that increase, and an accentuated response to drugs that decrease, the functional activity of the catecholamines. However, there is a dearth of information on the behavioral responsiveness to other drugs when administered at this age and of the possible mechanisms of the altered peri-adolescent psychopharmacological response. The present research is designed to more extensively examine the nature of this altered peri-adolescent responsiveness--to examine the psychopharmacological sensitivity to peptides and drugs affecting other neurotransmitter systems, and to assess the possible hormonal role in this altered peri-adolescent drug response. Results of our work under this grant in the past year suggest that the altered sensitivity to drugs during the peri-adolescent period does not appear to be a result of an altered pattern of drug distribution in the brain of animals at this age. Moreover, the altered pattern of drug responsiveness seen at this age with catecholaminergic agonists and antagonists is different from that seen with morphine (where sex differences are prominent), and may be related to the maturation of dopamine autoreceptors in certain brain regions.